This invention relates to mordenite zeolites and their use as catalysts in the alkylation of polynuclear aromatic compounds to alkylates enriched in the para-substituted isomers.
The para,para'-dialkylates (p,p'-dialkylates) of polynuclear aromatic hydrocarbons, such as 4,4'-dialkylated biphenyl or 2,6'-dialkylated naphthalene, are valuable intermediates in the preparation of monomers from which thermotropic liquid crystal polymers are synthesized. Liquid crystal polymers are high molecular weight polymers which naturally exist in or can form liquid-crystal states. The liquid-crystal state is a highly anisotropic fluid state which possesses some properties of a solid and some properties of a conventional, isotropic liquid. For example, the typical liquid crystal flows like a fluid, while retaining much of the solid state molecular order. Thermotropic liquid crystals refer to those liquid crystals which are formed by the adjustment of temperature. Generally, for a molecule to possess a liquid-crystal state the molecule must be elongated and narrow, and the forces of attraction between these molecules must be strong enough for an ordered, parallel arrangement to be maintained after melting of the solid. Thus, bulky substituents positioned anywhere but on the ends of an elongated molecule will usually destroy the liquid-crystal state. Accordingly, p,p'-disubstituted aromatic compounds are likely to exhibit liquid crystalline properties, whereas meta- and ortho-disubstituted aromatic compounds are not. Thermotropic, liquid crystal polymers find utility in the formation of ultra high-strength fibers and films. An overview of liquid crystals may be found in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Volume 14, John Wiley & Sons, New York, N.Y., pp. 395-427.
One group of monomers from which thermotropic liquid-crystal polymers are synthesized is the p,p'-dihydroxy polynuclear aromatics. Phenol, for example, is dialkylated at the ortho positions with isobutylene, and the resulting dialkylated phenol is coupled at the para position to form 3,3'5,5'-tetra(t-butyl)-4,4'-dihydroxybiphenyl. (See U.S. Pat. No. 4,108,908.) This substituted biphenyl is dealkylated to yield p,p'-dihydroxybiphenyl which reacts with aromatic diacids and hydroxy acids to form liquid crystal polymers. Aromatic diacids are also prepared in a multi-step process. p-Chlorotoluene, for example, is coupled to form 4,4'-dimethylbiphenyl, which is subsequently oxidized to 4,4'-biphenyldicarboxylic acid. (See U.S. Pat. No. 4,263,466.)
As illustrated in the examples hereinbefore, the syntheses of dihydroxy polynuclear aromatics and diacids require considerable effort. An alternate route based on the direct alkylation of polynuclear aromatics would require fewer starting materials and fewer steps. For example, if biphenyl could be dialkylated with propylene selectively to p,p'-(diisopropyl)biphenyl, the latter could be converted directly to p,p'-dihydroxybiphenyl or to p,p'-biphenyldicarboxylic acid. Thus, the selective alkylation of polynuclear aromatic compounds would greatly simplify the syntheses of dihydroxyl polynuclear aromatics, diacids and hydroxy acids which are the building blocks for liquid crystal polymers.
It is known that aromatic hydrocarbons can be alkylated in the presence of acid-treated zeolite. U.S. Pat. No. 3,140,253 (1964) and U.S. Pat. No. 3,367,884 (1968) broadly teach the use of acid-treated mordenite for the alkylation of aromatic compounds. However, such alkylations are generally not selective with respect to site and number of substitutions.
More specifically, some of the prior art illustrates the use of acid-treated zeolites in the alkylation of polycyclic aromatic compounds. For example, U.S. Pat. No. 3,251,897 teaches the alkylation of naphthalene by acid-treated zeolites X, Y, and mordenite. However, the conversion of naphthalene is shown to be low, and the selectivity to di- and triisopropyl naphthalenes is low and otherwise unspecified. Japanese Pat. No. 56-156,222 (1981) teaches the alkylation of biphenyl using silica alumina catalysts to give the monoalkylate in a para/meta ratio of 3/2. U.S. Pat. No. 4,480,142 (1984) discloses the alkylation of biphenyl in the presence of an acid-treated montmorillonite clay to yield 2-alkylbiphenyls as the major product.
Some of the prior art describes the use of acid-treated zeolites for the preparation of dialkylates high in para isomer content. For example, Japanese Pat. Nos. 56-133,224 (1981) and 58-159,427 (1983) teach the use of acid extracted mordenite for the gas phase alkylation of benzene or monoalkylbenzenes to p-dialkylbenzenes. U.S. Pat. No. 4,283,573 (1981) discloses the alkylation of phenols by use of H-mordenites to produce p-alkyl phenols with placement of the phenolic moiety at the 2-position of the alkyl chain. U.S. Pat. No. 4,361,713 (1982) describes the treatment of numerous ZSM zeolite catalysts with a halogen-containing molecule, such as HCl, or CCl4, and calcination at a temperature of from 300.degree. C. to 600.degree. C. to enhance the para-selective properties of such catalysts in the alkylation of benzene compounds. As illustrated with toluene, the conversion is taught to be low, while the selectivity to p-xylene is taught to be high.
Most recently, European patent application Ser. No. 0-202,752 (1986) teaches the alkylation of multi-ring aromatic hydrocarbons to alkylated derivatives high in .beta. and .beta.,.beta.' isomers. The process involves contacting a multi-ring aromatic hydrocarbon with an alkylating agent other than an alcohol, such as an alkylaromatic hydrocarbon, in the presence of a medium- or large-pore, acid-treated zeolite.
Despite the numerous teachings in the prior art, there are few useful results of the alkylation of polycyclic aromatic compounds by zeolite catalysts. Such alkylations tend to give low conversion of the polynuclear aromatic compound, and a low yield of the desirable p,p'-dialkylates or linear alkylates. A variety of by-products of low value is produced, making the separation and isolation of products difficult, if not impractical.
It would be highly desirable to find a process for the alkylation of polycyclic aromatic compounds which would give high yields of disubstituted polycyclic aromatic compounds enriched in the para alkylated isomers. Such a process would therefore find widespread utility in the synthesis of monomers for the preparation of thermotropic liquid crystal polymers.